The electrically driven valve of this kind comprises a valve spindle V having a valve head H that is seated on or moved away from a seat Z provided in a valve body D as exemplified in FIG. 12 and as disclosed in Japanese Patent No. 3181118. A bush P having an internal thread F and a bearing part J is fixed in the valve body D while fixed in a rotor R of a stepping motor M is a valve holder W having an external thread T to mesh with the internal thread F and a journal K to slide on the bearing part J.
The valve spindle V is assembled in the valve holder W and a fixing ring L is fit onto an open side. An upper part of the valve holder W is inserted into a receiving member G fixed on a rotor casing C, and a return spring U is interposed between the receiving member G and the rotor R to mesh the external thread T with the internal thread F after disengagement due to full opening of valve. A valve spindle urging spring Q is interposed between the valve holder W and the valve spindle V to urge the valve spindle V in the direction of closing the valve.
And predetermined exciting pulses are supplied to a stator E of the stepping motor M to rotate the rotor R regularly and irregularly so as to move the valve spindle forwards and backwards, thereby changing the sectional area of the opening between the valve seat Z and the valve head H to carry out the flow control.
In the above conventional art, upon closing the valve, the valve head H is pushed against the valve seat Z by the force of the valve spindle urging spring Q. Thus, when upstream and downstream sides with respect to the closed valve seat Z part are subjected, in the direction of opening the valve, to an excessively large differential pressure which results from a transitional change of refrigerant pressure or other causes and is larger than the urging force of the spindle-urging spring Q, such problem is caused that the valve spindle V is lifted and not able to keep closing the valve.
Closing the valve may employ, in place of the spindle-urging spring Q, a structure of fastening the valve spindle V onto the valve seat Z by use of screw threads directly provided on the valve spindle, in order to improve stability upon closing valve. For example, in FIG. 12, there may be not adopted the screwing between the bush P and the valve holder W but may use such features that the valve spindle V is directly provided on the outer periphery with a male thread part T which is meshed with a female thread F of the bush P, and the valve spindle V is rotated following the valve holder W at the rotor R side.
However, as shown in FIG. 13, upon initializing the electrically driven valve, the stator E of the stepping motor M is given the exciting pulses excessively and in the direction of closing the valve (FIG. 14). In this case, there is a fear that the stepping motor M steps out and the rotor R causes bringing back of the valve spindle, thereby causing the valve spindle to move up and down slightly.
From the position ({circle around (1)}) in FIG. 13 that defined at the moment of the valve's being closed following the rotor R turning counterclockwise to another position ({circle around (2)}) that defined by three pulses after the position ({circle around (1)}), N poles of the rotor R are attracted by S poles of the stator E moving away counterclockwise, so that the rotor R is subjected to a force in the direction of closing the valve. At the position ({circle around (3)}) defined by five pulses after the valve's being closed, N poles of the rotor R are attracted by S poles of the stator E turning clockwise to approach the N poles, and the rotor R is given a force in the direction of opening the valve. Thus, the rotor R turns regularly or irregularly in a certain extent of angle α ({circle around (4)}), and the valve spindle moves up and down with the screwing being slackened and tightened, resulting in that the state of the valve's being closed is not able to be kept.